The present invention relates to amplifier circuits for driving a load, and more particularly, to amplifier circuits for achieving maximum power transfer from a plurality of devices each having limited current capability.
It is often necessary to achieve a power output from a circuit beyond the capability of particular devices within the circuit. This is often the case when, for economic reasons, the lower power devices are readily available at a reasonable cost and upgrading to higher power devices requires an increased cost factor that is unreasonable with respect to the achievable power gain.
Paralleling lower power devices will increase power output capability. However the plurality of lower power devices must be matched so as to evenly distribute the contribution from each of the devices, otherwise, at high power outputs, some of the devices will load the other devices instead of contributing to driving the load. Additionally, the devices have to track each other at lower power output in order to achieve simultaneous maximum peak power from the devices.
Thus, it is desirable when using a plurality of cooperating devices for increasing the power output into a load, that each of the devices be identical. Such a goal is easier to achieve when all of the plurality of devices are present on a same integrated circuit chip. In such a situation they are in close physical proximity to each other and are produced at the same time under identical processes. Operational amplifiers are particularly useful for such parallel load driving purposes since often four or more closely matched devices are available on a single integrated circuit chip at a minimal cost. However, even though the devices are close to being identical, each of the devices requires external components which have tolerances which greatly effect the tolerance of the complete amplifier circuit.
In the prior art, another common configuration for increasing the power output to a load is to use two low power output devices such as shown in National Semiconductor linear applications handbook note AN69-5. This applications note shows two operational amplifiers operating in a push-pull configuration with the amplifiers being fed with 180 degree out-of-phase signals. This configuration provides twice the voltage swing across the load for a given supply voltage thus increasing the power output capability by a factor of 4 over a single amplifier. This will work for high impedance loads but it is not satisfactory for low impedance loads where the output devices are current limited rather than voltage limited.
When the lower power devices are current limited, it is common to have a single device driving an external current amplifier in order to boost the power delivered to the load. In such a situation, an operational amplifier will often drive a pair of external NPN and PNP transistors connected between positive and negative supply voltages in what is called a "totem pole" configuration such as shown in National Semiconductor applications note 125 in FIG. 15 and note 127 in FIG. 7. However, as explained above, these external devices, aside from being discrete devices and therefore not necessarily closely matched, present an unreasonable increase in cost, especially if only a moderate increase in power output is required.
Another possible configuration includes two operational amplifiers with their non-inverting input terminals coupled in parallel with each other to a signal source and their outputs coupled in parallel to a load. Each of the operational amplifiers has a first feedback resistor coupled between the common output at the load and an inverting input, and a second feedback resistor coupled between the inverting input and signal ground. The gain of each amplifier is the ratio of the respective first and second feedback resistors. The outputs are coupled in parallel through isolation resistors so that the low output impedances of the amplifiers do not load each other. Such a configuration has the disadvantage that even if the two operational amplifiers are reasonably identical, e.g., because they are on the same integrated circuit chip, the tolerances of the gain determining feedback resistors and other external components must be taken into account. E.g., if the resistors are five percent tolerance resistors, which is commonly case for consumer products such as television receivers, the possible difference in the gain just between two amplifiers can be as great as ten percent. This means that one amplifier can clip substantially before the other. Since power is proportinal to the square of the voltage applied to the load, this gain difference translates to a twenty percent difference in power delivered to the load. The resistance ratio differential can be reduced by using high tolerance components, e.g., one percent tolerance resistors. This may be an acceptable solution for instrumentation devices but presents an undesirable cost for consumer products in a highly competitive field such as television receivers.
Accordingly, it is desirable to achieve the maximum power output from a plurality of devices having relatively limited current sourcing capabilities in a cost effective manner.